It is a remarkable fact that planets start out as microscopic
grains within the protoplanetary disks of gas and dust in orbit
around newly-formed protostars, somehow growing by a factor of
$10^{40}$ in mass in a period no more than $10^7$ years. In the
early stages of the planet formation process, small dust grains
settle into the midplane of the disk in a few thousand years. As
the dust layer gets thinner and denser, a vertical shear develops
between the dust-rich layer at the midplane and the dust-poor gas
above and below this layer. Of great interest is under what
conditions such a layer will be unstable to Kelvin-Helmholtz
instability (KHI), which will remix the dust with the gas,
thwarting the formation of planets. In our previous work, we
worked in the single-fluid limit in which the local dust-to gas
ratio was an advectively conserved quantity (valid when the
dust-gas friction time is very short). Here, we present new
simulation in which this assumption is relaxed. We employ
2-fluid simulations of dust and gas to explore the evolution of a
dust layer in the more general case in which the dust grains and
gas can slip through each other. We will describe conditions
that allow the dust layer to settle to sufficient density to
gravitationally clump-up to form planetesimals before the onset
of KHI.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2010.DFD.CV.9